1. Field of the Invention
This invention relates to a method and a device for stabilizing photomultiplier tubes of a radiation image device, in particular scintillation gamma camera against drift by illuminating the photomultiplier tubes with light.
2. Description of the Prior Art
It is known that the gain of photomultiplier tubes (PMT's) changes with time. Due to this, PMT's have to be retuned very often. Automatic retuning devices which utilize an additional light peak for retuning are for example described in the European Patent Application 0,066,763 and U.S. Pat. Nos. 3,900,731 and 4,047,034. In the case of the European Patent Application No. 0,066,763 the light peak is produced by an array of pulsed light emitting diodes, positioned close to the cathode ends of the PMT's. In the case of U.S. Pat. No. 3,900,731, the light peak is produced by a pulsed light emitting diode which is positioned in a light conductivity disc in front of the cathode end of a PMT and in the case of U.S. Pat. No. 4,047,034 a light peak produced by a pulsed light emitting diode is transferred to the cathode ends of the PMT's of an PMT array via a bundle of optical fibers. Another retuning method and device is described in the commonly owned, copending application Ser. No. 462,839 of Everett W. Stoub, entitled "A Method and Circuit for Stabilizing Conversion Gain of Radiation Detectors of a Radiation Detection System", which was filed on Feb. 2, 1983. For each radiation detector tuning point the energy map status is measured in parallel. Thereby an energy response vector is obtained, which is multiplied with a predetermined deconvolution matrix. The deconvolution matrix is the inversion of a contribution matrix containing matrix elements C.sub.ij, each such matrix element C.sub.ij representing the relative contribution level of a radiation detector j of said detection system for a point radiation source placed at a location i. Thus a calculated gain vector is obtained with respect to which the gains of the radiation detectors are adjusted such that an ideal gain vector is obtained.
All these retuning methods and devices work only accurately when the magnitude of gain change lies within a certain range. At the very beginning of their life time PMT's drift most rapidly. Therefore, the dynamic tuning range of the aforementioned methods and devices can be exceeded within the first few months of operation. Due to this either manual adjustment or replacement of PMT's is required. This is very costly and time consuming.
It is known that PMT's can artifically be altered by illuminating with light. However, if at all, PMT's are "bright aged" by PMT manufacturers by arranging bundles of PMT's in a special room and illuminating them for a certain time period with a diffuse incandescent light source. As a result it should be expected that all illuminated PMT's have the same degree of "bright aging". However, as practice shows most of the "bright aged" PMT's after arranged in an array for final application will not have magnitudes of gain changes which lie in the range, in which the aforementioned retuning methods and devices work accurately.